How to use Shader Editor for Grease Pencil Strokes and Fills
You can't, found this on the official documentation.


  The shader is not yet a BSDF capable shader to interact with Blender lights and can only be setting up on the Material Properties panel (it is not a shader node).
  https://docs.blender.org/manual/en/latest/grease_pencil/materials/grease_pencil_shader.html


Hope they add it soon.